The invention relates generally to a gas analyzer for determining the concentration of the oxides of nitrogen in a sample gas and more particularly is directed to a chemiluminescence NO, NO.sub.2, NO.sub.x analyzer for determining the concentration of the oxides of nitrogen in the exhaust gas of a combustion engine or power plant.
While there are five oxides of nitrogen, there are only two that are of primary concern with regard to the emissions from a combustion engine; namely, NO (nitric oxide) and NO.sub.2 (nitrogen dioxide). The total of the emissions of the oxides of nitrogen is generally referred to as NO.sub.x. Existing and forthcoming legislative measures both in the U.S. and Europe have created a need for an inexpensive and accurate analyzer for monitoring the oxides of nitrogen emissions of automotive engines. Similarly, there is much interest in monitoring the emissions from stationary power plants, or the like, and monitoring ambient concentrations of the oxides of nitrogen. Most of the NO.sub.x emitted by gasoline engines is NO which slowly oxides to NO.sub.2. However, in some combustion engines, such as in a diesel engine where compression ratios are higher and air fuel ratios are leaner, a substantial amount of NO.sub.2 is formed directly during the combustion process.
There are a number of possible techniques for measuring NO concentrations. These include nondispersive, infrared or ultraviolet gas analysis. However, infrared gas analysis of NO is difficult because the absorptivity of NO lies in a range where there is interference with water vapor. While NO has a very strong ultraviolet absorption line where water vapor would not act as a contaminate, nondispersive ultraviolet analysis has not been successful because a very selective source of ultraviolet energy is required and the sources which have been developed to date have a very short life span. The most popular technique for measuring NO in the prior art involves the principle of chemiluminescence. Chemiluminescence involves the oxidation of NO to N.sub.2 instantaneously with O.sub.3 (ozone). When this occurs, the NO.sub.2 which is formed is in an excited state and it immediately returns to its ground state giving off a photon. The photon emission of the NO.sub.2 returning to its ground state is proportional to the amount of NO in the sample gas as long as stoichiometric or greater quantities of ozone are present. The reaction takes place in approximately 10 milliseconds and for practical purposes is considered instantaneous. Thus, gas analyzers are found in the prior art which measure this chemiluminescent reaction with a photomultiplier for the purpose of producing a signal which is representative of the NO concentration in a sample gas.
In fact, the use of chemiluminescent nitric oxide detectors has become widespread in the prior art. The typical applications for such detectors are in air pollution monitoring instruments and gas analyzers for determining atmospheric concentrations of the oxides of nitrogen or the concentrations of the oxides of nitrogen in auto gas emissions, power plant emissions, etc. The success of prior art chemiluminescent detectors has almost lead to the adoption of such instruments as de facto legislative standards. However, these prior art chemiluminescent oxides of nitrogen gas analyzers have inherent problems which stem from the use of a photomultiplier for measuring the chemiluminescent reaction. Photomultipliers are vacuum tube devices which are large, fragile and expensive. It is generally difficult to supply such a tube with adequate air flow for cooling and lowering the dark current while at the same time meeting shielding requirements with regard to ambient light and radio frequency energy which substantially interfere with the operation of the device. In some prior art analyzers of this type, a thermoelectric cooled photomultiplier tube is used. While this results in an instrument having good performance, the cost of the instrument is high. Still further, although the gain of a photomultiplier tube is high, because the tube comprises a plurality of plates arrayed within a glass envelope, it is difficult to place the detector plates within close proximity to the chemiluminescent reaction. Since the photons issuing from the chemiluminescent reaction are very dispersive and difficult to focus, this can have a deleterious effect on detector sensitivity. Other problems with prior art chemiluminescent detectors in general relate to the fact that the sample gas under investigation normally contains large amounts of CO.sub.2 which has a quenching effect on the chemiluminescent reaction. The ozone requirements of these instruments is relatively high, and ozone is itself a noxious gas. The range of these instruments can be somewhat limited, and in cases where a hot gas sample is drawn from exhaust of a combustion engine, water condensate can interfere with the operation of the instrument.